We propose to use electron spin resonance to search for free radical metabolites of polycyclic hydrocarbons and dibenzodioxins. Free radicals of electron donor molecules are formed in the peroxidase reaction according to the following mechanism. Peroxidase plus H2O2 yields Compound I plus H2O. Compound I plus R yields Compound II plus R ion. Compound II plus R yields Peroxidase plus R ion. Many compounds are known to undergo such a one-electron oxidation in the presence of a variety of peroxidases and hydrogen peroxide. Electron spin resonance spectra have been obtained of the free radical metabolites of hydroquinone and N-hydroxy2-acetylaminofluorene and many biological compounds. This class of enzymes appears to have little substrate specificity, any electron donor with a sufficiently low oxidation potential should be metabolized to a free radical. In addition to lactoperoxidase and myeloperoxidase, hematin, methemoglobin and cytochrome P-420 all have peroxidase activity.